Kerala Syllabus Class 8 Basic Science: Chapter 14 Water - Questions and Answers
Std 8: Physics: Chapter 14: Water: Questions and Answers
♦ What are the uses of water?
• For drinking
• For bathing
• For cooking
• Washing
• Cleaning
• Irrrigation
• Hydroelectric power generation
• Industrial purposes
♦ Solubility
♦ Experiment
Take some water in different beakers. Add a little salt to one beaker, sugar to another, honey to the third, vinegar to the fourth and Caustic Soda (NaOH) to the fifth. Test the water in these beakers using blue and red
litmus paper. Record the experiment results below.
| Pure Water / Substance Added to Water | Blue / Red Litmus Paper Experiment – Observation | Inference |
|---|---|---|
| Pure Water | No change in the colour of the litmus paper | Water has no acidic or basic properties. |
| Common Salt | No change in the colour of the litmus paper | Water has no acidic or basic properties. |
| Sugar | No change in the colour of the litmus paper | Water has no acidic or basic properties. |
| Honey | Blue litmus turns red | Water acquires acidic properties. |
| Vinegar | Blue litmus turns red | Water acquires acidic properties. |
| Soap powder | Red litmus turns blue | Water acquires basic properties. |
| Lime juice | Blue litmus turns red | Water acquires acidic properties. |
| Sodium hydroxide (Alkali) | Red litmus turns blue | Water acquires acidic properties. |
♦ Hard Water and Soft Water
If the soap does not lather well, that water is called hard water. If the soap lathers well, that water is called soft water.
♦ Why is water called the universal solvent?
Water is called the universal solvent because it can dissolve a wide variety of substances.
♦ Reason for the hardness of water.
As water flows through the soil, many salts from the soil dissolve in it. These dissolved salts make the water hard. The hardness of water is caused by some salts of metals like magnesium and calcium.
♦ Experiment
Take some water in a test tube and dissolve magnesium bicarbonate or calcium bicarbonate in it. Then, try to form a lather with soap in the solution.
♦ Complete the table.
| Substance Added to Water | Soap lathers well/ Doesn’t lather well | Conclusion |
|---|---|---|
| Pure water (nothing added) | Soap lathers well | No hardness / soft water |
| Magnesium bicarbonate/ Calcium bicarbonate. | Soap Doesn’t lather well | Hard water |
Vinegar: Acidity
Water: …………….
Answer: Neutral
♦ What is the temporary hardness of water?
Temporary hardness is caused by bicarbonates of calcium and magnesium. When such water is boiled, the hardness is removed.
♦ What is the permanent hardness of water?
Salts like sulphates or chlorides of metals such as magnesium or calcium are dissolved in water; their hardness cannot be removed by boiling. This type of hardness is called permanent hardness.
♦ Which of the following substances is responsible for the permanent hardness of water?
Magnesium sulphate, magnesium bisulphate, ice, calcium bicarbonate
Answer: Magnesium sulphate
♦ Some statements related to water are given below. Analyse them and choose the correct answer.
(i) The nature of water depends on the substance dissolved in it.
(ii) Water dissolved in calcium bicarbonate is hard water.
(iii) Water is in liquid state from 0°C to 100°C.
(iv) The boiling point of water decreases as pressure increases.
(v). The force of attraction between water molecules is greater than the force of attraction between water and glass.
(a) Statements (i), (ii), and (v) are true but statements (iii) and (iv) are false.
(b) Statements (i), (ii), and (iii) are true but statements (iv), and (v) are false.
(c) Statements (i), (ii), (iii), (iv), and (v) are false.
(d) Statement (i) is true but statements (ii), (iii), (iv), and (v) are false.
Answer: (b) Statements (i), (ii), and (iii) are true but statements (iv), and (v) are false.
♦ The following is an observation of an experiment conducted using water samples taken from three sources. Analyze these and answer the questions
Sample A: When soap is added and stirred it lather well.
Sample B: When soap is added and stirred it does not lather well.
Sample C: When soap is added and stirred it does not lather well. After boiling the soap lather well.
a) Which of these waters has temporary hardness?
b) Which salts give permanent hardness to water?
Answer:
a) Sample C has temporary hardness, because boiling removes the hardness and allows soap to lather.
b) Permanent hardness is caused by salts like sulphates and chlorides of calcium and magnesium.
♦ Surface Tension
Due to the mutual attraction between water molecules, a force acts on the water surface, making its surface area as small as possible. This force is called surface tension.
♦ Analyze the statements given below and choose the correct answer.
Statement (A): There is no change in temperature until water at 100°C completely turns into gas.
Reason (R): The heat gained by water at 100°C is completely used for the change of state.
a) Statements (A) and (R) are correct, but (R) is not the correct explanation of (A).
b) Statements (A) and (R) are correct, but (R) is the correct explanation of (A).
c) Statements (A) and (R) are incorrect.
d) Statement (A) is correct, but (R) is incorrect.
Answer: b) Statements (A) and (R) are correct, but (R) is the correct explanation of (A).
♦ Take a tumbler filled with water. Gently place a blade on the surface of the water. Can you explain why the blade stays on top without sinking? Now try placing a needle on the surface of the water. Can it stay without sinking?
Floating of a blade on the surface of water, walking of some insects on the surface of water, etc., are possible due to the surface tension of water.
As in Figure 14.5 (c), the molecules at the surface are attracted sideways and downwards only. So the water surface behaves like a stretched film. This is the main cause of surface tension. This is why a blade and needle can float, and some insects can walk on water.
♦ Small insects can walk on the surface of water. What is the force felt on the surface of the water that causes this?
surface tension.
♦ Water can wet the surface of a plastic sheet but not the surface of a Colocasia leaf. Why?
If water is dropped on some leaves, like the Colocasia leaf, it does not adhere to them. The reason is that the molecules in the leaf and the water molecules have very little attraction between them.
♦ Water can rise up through thin glass tubes. Why?
This happens because the attractive force between glass and water is stronger than the attractive force between water molecules.
♦ It is easier to clean dirty clothes with soap and hot water. Why?
The surface tension of water can be reduced by adding soap to it or heating the water. This is used for washing clothes. If the clothes are washed in hot water or soap water, the surface tension is decreased, and it helps clothes wet better and clean more easily.
♦ Boiling Point
♦ Experiment
As shown in the figure, fill a flask halfway with water and fix it on a stand. Close the flask with a two-holed cork. Insert a thermometer so that its bulb is touching the water. Note the initial temperature. Now heat the water.
When the thermometer reading (temperature) reaches 100°C, bubbles are formed from all parts, and water starts boiling. If heating is continued, the thermometer reading remains stable without no rise.
♦ Why doesn’t the temperature rise once the water starts boiling?
When water starts to boil, the water molecules begin to change from liquid to gas. The extra heat energy is used to overcome the force of attraction between the molecules. In other words, the heat is used for the change of state, not to increase the temperature.
♦ Which contains more energy, boiling water or steam?
For steam. Temperature does not rise at boiling even if heat is given. But the excess heat given will be stored in vapour molecules because steam posses more energy than water molecules.
♦ Which can cause severe burns? Boiling water or steam? Discuss.
Steam. As steam posses more heat energy than boiling water, the burn due to steam is more severe than boiling water.
♦ Pressure and Boiling Point
♦ Experiment
syringe. Water in the tube starts boiling again. The reason is that when pressure decreases, the boiling point decreases.
♦ Why does cooking in open vessels take more time in high mountain regions?
Atmospheric pressure will be lower at high altitudes than at sea level. So at high mountain regions, water boils below 100°C in open vessels. So it took more time to cook food there. (Once boiling begins, heat energy is used for phase transition. So food particles would not get heat energy.)
♦ What change happens to the boiling point when pressure increases? How does cooking become easier in pressure cookers?
When pressure cookers are closed and heated, steam cannot escape from them. As a result, the pressure inside increases. When pressure increases, the boiling point also increases. This made cooking easy in a pressure cooker.
♦ Heat capacity
Heat capacity is the amount of heat energy required to raise the temperature of a given substance by 1°C.
♦ Heat Capacity of Water
♦ Experiment
Take two similar beakers. In one, pour water, and in the other, take coconut oil of same mass. Place both of them in a water bath and heat them. Stir the liquids in the beakers continuously. Record the temperature
at regular time intervals.
Observation: Even if the same amount of heat is given, the temperature of coconut oil is greater than that of water at each time.
Reason: Water has a greater heat capacity than coconut oil. Terefore 1°C rise in temperature, water needs more heat energy than coconut oil.
Water is a liquid with a very high heat capacity. So, the temperature of the water does not rise quickly.
♦ When the same amount of heat is given to water and coconut oil, which one’s temperature rises faster in a fixed time? Why?
Even if the same amount of heat is given, the temperature of coconut oil is greater than that of water at each time. Water has a greater heat capacity than coconut oil. Terefore 1°C rise in temperature, water needs more heat energy than coconut oil.
♦ How can we make use of high heat capacity of water?
• Water is used to cool hot objects.
• In the radiators of vehicles, water is used to cool the engine
• Water helps in maintaining the body temperature within a limit.
♦ What is the composition of a water molecule?
A water molecule is made up of two hydrogen atoms and one oxygen atom (H₂O).
♦ Why is water considered a precious natural resource?
Water made life possible on Earth and is essential for all living organisms.
♦ In how many states can water exist naturally?
Water can exist in all three states — solid (ice), liquid (water), and gas (steam/vapour).
♦ Does water retain the individual properties of hydrogen and oxygen?
No. Even though water is formed by the combination of hydrogen and oxygen, the individual properties of hydrogen or oxygen are not maintained in water. It has its own unique physical and chemical properties.
♦ What could be the reason for water to occur in the liquid state?
There is a special kind of attractive force between water molecules. This force is the reason for many special properties of water. This attractive force helps water to keep it in liquid form at room temperature. It is the reason for the surface tension of water. If there is no such attractive force, water will be a gas at atmospheric temperature.
♦ What would happen if the attractive force between water molecules did not exist?
Water would stay only in the gaseous state. There would be no rivers, no seas, and probably no living things on earth.
♦ Water is formed by combining hydrogen and oxygen. Is it possible to separate water back into hydrogen and oxygen?
Water can be split into hydrogen and oxygen by passing electricity through it. This process is called electrolysis.
♦ Water can be separated by electrolysis.
a) Write an example of an apparatus that can be used to perform electrolysis of water.
b) What are the gases produced by electrolysis of water?
c) Which gas is produced in large quantities?
Answer:
a) Hoffman water voltameter
b) Hydrogen and oxygen
c) Oxygen
♦ Ice floats in water. Why?
When water turns into ice, its volume increases. Therefore, its density decreases. As the density of ice is low, it floats in water.
♦ In cold countries, temperatures often drop below zero degrees Celsius. What if the water in rivers and lakes there turns completely into ice? Wouldn’t aquatic life get trapped in the ice and die? Does that happen? What could be the reason?
The lower density of ice compared to water is helpful for aquatic life. In cold countries, when the temperature drops below 0°C, water in lakes and rivers becomes ice. As this ice floats on water fishes and other aquatic animals can live in the water below it. The heat insulation property of ice prevents the further freezing of water also.
♦ Take the same amount of water in two beakers. Dissolve some salt in one of them. Find the boiling point of the water in each beaker using a thermometer. What difference is there in the boiling point of the water with salt added?
The boiling point of water with added substances such as salt is higher than the boiling point of pure water. When salt is added, more energy is required to boil the water. That is why the boiling point of salt water is higher than that of pure water.
♦ Our rivers, streams, and seas are getting polluted. What could be the reasons for this?
• Chemicals from agricultural fields
• Chemicals from factories
• Oils from vehicles
• Food wastes
• Hospital wastes
• Pesticides
• Soaps and detergents
• Biological residues
♦ What are the main problems caused by water pollution?
• Destruction of aquatic plants
• Damage to the food chain
• Soil pollution
• Diseases
• Shortage of drinking water
Let’s Assess
1. What changes will occur on heating water that has already reached boiling point?
Answer: Temperature does not rise as the heat given off after boiling is used for phase transition.
2. Explain how the boiling point of water varies under the following conditions:
a. In high-altitude areas
b. At sea level
c. In vacuum containers
d. In pressure cookers
Answer:
a) Decreases
b) Normal boiling point
c) Decreases
d) Increases
3. Due to the intermolecular attraction between water molecules, a force arises that tends to minimize the surface area of water. How can this be demonstrated?
Answer:
Experiment with soap film
Tie a thread across a light metal ring. Dip the ring into soap water to form a soap film. Now, gently break the soap film on one side of the thread using a pin. The thread is attracted to the other side so as to reduce the surface area of the soap film.
4. Write any three special properties of water and explain situations in daily life where we make use of these properties.
Answer:
• Solubility - In daily life, water is taken as a solvent for many substances and also for diluting substances.
• Boiling point - The comparatively high boiling point of water helps cook food.
• Heat capacity - Water is used for cooling in the radiators of vehicles because of its high heat capacity.
5. Observe the picture.
Answer:
The attractive force between water molecules may be less or greater than the attractive force between water and other substances. If water and glass come in contact, as the attraction between glass and water exceeds that between water molecules, water rises in the glass tubes.
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